This will make a number of things easier in the future, as well as (finally!)
avoiding the Id-smashing problem which has plagued developers for so long.
Boy, I'm glad we're not using sup anymore. This update would have been
insane otherwise.
if_sl and if_ppp (from ppp-2.2), eliminating the nearly identical
pppcompress.[ch] code. Add maximum VJ compression states argument to
sl_compress_init().
if_sl: call sl_compress_init() with the extra argument.
